1. Field of the Invention
This application relates in general to a piezoresistive material and in particular to a piezoresistive material having a micro-deformable rough texture surface.
2. Description of the Related Art
Conventional piezoresistive materials are elastic. Resistance of the piezoresistive material can be calculated by the equation R=ρ*L/A, wherein R is the electrical resistance of a uniform specimen of the material (measured in ohms, Ω), ρ is the resistivity of the material (measured in ohms-centimeters, Ω-cm), L is the length of the piece of material (measured in centimeters, cm), and A is the cross-sectional area of the specimen (measured in square centimeters, cm2). According to the equation R=ρ*L/A, the resistance R of the piezoresistive material increases with the decrease of the length/thickness L.
When an external pressure, exerted on the piezoresistive material, is released, the piezoresistive material can recover to an original state by viscoelasticity thereof for a period. However, the piezoresistive material could result in hysteresis and inaccurate pressure measurement after repeated operations.
Referring to FIG. 1, a conventional pressure sensor comprises two substrates 10 and 109 disposed on top and bottom sides therefore. Two metal electrodes 11 and 119 are disposed on the substrates 10 and 109, and two piezoresistive layers 12 and 129 are respectively disposed on the metal electrodes 11 and 119 with a space 16 formed therebetween. A supporter 15 is disposed between the substrates 10 and 109 to fix the two piezoresistive layers 12 and 129, so as to form the space 16 therebetween. As shown in FIG. 1, the metal electrodes 11 and 119 are electrically connected to a circuit system 13. When pressure P is applied to the pressure sensor, as shown in FIG. 2, the piezoresistive layer 12 deforms downwardly and contacts the piezoresistive layer 129, wherein the piezoresistive layers 12 and 129 have a total thickness of L1. Hence, an output resistance R1 of the pressure sensor can be determined by the equation R1=p*L1/A.
When pressure sensor is further pressed and deformed as shown in FIG. 3, the total thickness of the piezoresistive layers 12 and 129 is reduced to L2. In this state, an output resistance R2 of the pressure sensor can be determined by the equation R2=ρ*L2/A.
FIG. 4A is a pressure-resistance diagram in accordance with the pressure sensor of FIG. 1. The curve C1 in FIG. 4A illustrates the pressure-resistance characteristics when pressure is applied to the pressure sensor and increased from 1 to 10 psi. Additionally, the curve C2 in FIG. 4A illustrates the pressure-resistance characteristics when pressure is released and decreased from 10 to 1 psi.
Referring to the curve C1, when pressure increases to 2, 4, 6, 8 and 10 psi, the output resistance respectively decreases to 6.69*103, 3.86*103, 2.95*103, 2.54*103, and 2.27*103Ω. However, as shown in the curve C2, when pressure is released and decreases from 10 psi to 8, 6, 4, and 2 psi, the output resistance increases from 2.22*103 to 2.31*103, 2.53*103, 2.96*103 and 4.64*103Ω. The hysteresis phenomena of the pressure-resistance characteristics in conventional piezoresistive materials can be easily observed in curves C1 and C2 shown in FIG. 4A.
FIG. 4B is a pressure-conductivity diagram in accordance with the pressure sensor of FIG. 1, wherein the conductivity is the inverse of resistivity. The curve C3 in FIG. 4B illustrates pressure-conductivity characteristics when pressure is applied to the pressure sensor and increased from 0 to 10 psi. Additionally, the curve C4 in FIG. 4B illustrates pressure-resistance characteristics when pressure is released and decreased from 10 to 0 psi.
As the curve C3 shown in FIG. 4B, when pressure is 2, 4, 6, 8 and 10 psi, the conductivity is 1.49*102, 2.59*102, 3.39*102, 3.94*102, and 4.41*102 μsiemens, respectively. When pressure decreases from 10 psi to 8, 6, 4, and 2 psi, as the curve C3 shown in FIG. 4B, the conductivity decreases from 4.50*102 μ-siemens to 4.33*102, 3.95*102, 3.38*102 and 2.16*102 μ-siemens, respectively. The hysteresis phenomena of the piezoresistive materials are obvious and usually occur in conventional pressure sensors as the curves C3 and C4 show in FIG. 4B.